ABSTRACT
The thyroid hormones have profound effects on the development of neuromuscular system. These hormones exert their influence on both muscle fibers and related motoneurons during development. The masseter is one of the most important muscles for mastication in mammals. We attempted to evaluate the effect of thyroid hormone deficiency on the morphological characteristics of masseteric motoneurons in the period of alteration from sucking to biting and chewing in the rat Material and To induce hypothyroidism, timed pregnant dams received 50 ppm antithyroid drug propylthiouracil [PTU] in their drinking water and PTU was administered to the pups during suckling period. Horseradish peroxidase [HRP] was injected into the masseter [0.5-5 mecro lit, 40%] of normal and prenatal hypothyroid pups on postnatal days of 1, 5, 13, and 21 [n=24]. 24 to 48 hours later, following transcardial perfusion and fixation the brainstem blocks were cut to 50 micro m thick sections. After TMB histochemical reaction the morphological characteristics of HRP labeled motoneurons and their HRP labeling intensity was evaluated. Student's t-test and two-way analysis of variance [ANOVA] were used for statistical analysis.No significant morphological differences were observed at the end of first week of life. On day 15, hypothyroid labeled masseteric motoneurons consisted of 70% small and 30% medium neurons versus 40% and 60% in normal pups respectively [p<0.05].At the time of weaning, the number of large motoneurons dropped to 30% of normal value [p<0.001] with few, short, and disoriented dendrites The alteration in particular patterns of masseteric motoneuron morphology and a severe delay in size transition could affect the development and plasticity of oral motor behavior under congenital hypothyroidism